Since energy issues and global environmental issues are becoming more serious, solar cells are receiving more attention as an alternative energy for replacing fossil fuels. In the solar cell, carriers (electrons and holes) generated by light irradiation to a photoelectric conversion section composed of a semiconductor junction or the like are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit.
For example, in a crystalline silicon-based solar cell using a single crystal silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of fine metal is provided on a light-receiving surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, collecting electrode(s) are provided on the transparent electrode layer(s).
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste by a screen printing method. This method is simple in terms of the process itself, but has such a problem that the material cost of silver is high, and the resistivity of the collecting electrode increases when a silver paste material containing a resin is used. For decreasing the resistivity of the collecting electrode formed of a silver paste, it is necessary to thickly print the silver paste. However, since the line width of the electrode increases with the increase of the print thickness, thinning of the electrode is difficult, and the shading loss by the collecting electrode increases.
For solving these problems, a method is known in which a collecting electrode is formed by a plating method excellent in terms of material cost and process cost. For example, Patent Documents 1 and 2 disclose a solar cell in which a metal layer made of copper or the like is formed by a plating method on a transparent electrode that forms a photoelectric conversion section. In this method, first, a resist material layer (insulating layer) having an opening corresponding to the shape of a collecting electrode is formed on the transparent electrode layer of the photoelectric conversion section, and a metal layer is formed at the resist opening section of the transparent electrode layer by electroplating. Thereafter, the resist is removed to form a collecting electrode having a predetermined shape.
Patent Document 3 discloses a method in which an insulating layer of SiO2 or the like is provided on a transparent electrode, a groove extending through the insulating layer is then provided to expose the surface or side surface of the transparent electrode layer, and a metal collecting electrode is formed so as to be in conduction with an exposed area of the transparent electrode. Specifically, a method is proposed in which a metal seed is formed on the exposed area of the transparent electrode layer by a light induced plating method or the like, and a metal electrode is formed by electroplating with the metal seed as an origination point. In this method, the contact resistance between a transparent electrode layer and a collecting electrode can be lowered by providing a low-resistance metal seed.
Patent Document 4 discloses a method in which an electroconductive seed having a coarse roughness is formed on a transparent electrode layer and an insulating layer is deposited thereon. According to this method, the whole surface of a transparent electrode layer other than the electroconductive seed-formation part of the photoelectric conversion section is covered with the insulating layer, and discontinuous openings are formed in the insulating layer on the electroconductive seed. A metal layer is formed by plating through the openings in the insulating layer.
In formation of a photoelectric conversion section of a solar cell, thin-films such as a semiconductor layer, a transparent electrode layer, and a metal electrode layer are generally formed on the surface of a substrate by a plasma-enhanced chemical vapor deposition (CVD) method, a sputtering method or the like. These thin-films exist not only on the front surface of a substrate, but also on the side surface and on the back surface since the deposited films wrap around thereon, and thereby cause a short circuit and leakage between the front surface and the back surface. For preventing the above-described wraparound, for example, a method is proposed in Patent Document 5 in which semiconductor layers and electrode layers are formed while the periphery of a crystalline silicon substrate is covered with a deposition mask.
Patent Documents 6 and 7 disclose methods for removing the short circuit of the electrodes between the front surface and the back surface by performing predetermined processing after forming a semiconductor thin-film and an electrode on a substrate. Specifically, Patent Document 6 proposes a method in which a semiconductor thin-film and a transparent electrode layer formed on a crystalline silicon substrate are removed by laser irradiation to form a groove. In Patent Document 7, a method is disclosed in which a groove is formed by laser irradiation, and thereafter a crystalline silicon substrate is cleaved along the groove to form a solar cell in which the side surface of a photoelectric conversion section is composed of a cut surface. Either a semiconductor thin-film or an electrode layer is not present on the surface of the groove in Patent Document 6 and the cut surface in Patent Document 7, and therefore the problem of a short circuit resulting from wraparound is solved.